Hydraulic fracturing

ABSTRACT

Improvements in hydraulic fracturing of underground porous formations penetrated by a well bore are accomplished by the use of fracturing fluids comprising aqueous gels prepared from water or brines and certain polyacrylamides and related polymers.

1 Apr, 117, 11973 HYHMEUMULHC WAQ'HEUMNQG [75] lirWemtor: 1mm 1L.(31111111111111, Bartlesville.

[73] Assignee: Phiflips I Petroleum Company, Barflesville, Okla.

[22] Filed: Feb. 9, 11972 [21] Appi. No; 2241,9416

[5 6] Refieremw @ifred UNITED STATES FATE 3,254,719 6/1966 Root..166/308 Primary Examiner--Stephen J. Novosad Attorney-Quigg andOberlin No Dmws HYDRAULIC FRACTURHNG This invention relates to hydraulicfracturing.

Hydraulic fracturing of subterranean formations penetrated by a borehole has been widely employed for increasing the production ofhydrocarbon fluids, e.g., crude oil, natural gas, from said formations.Hydraulic fracturing comprises the injection of a fracturing fluid downa well penetrating a formation, and into said formation under sufficientpressure to overcome the pressure exerted by the overburden. Thisresults in creating a fracture in said formation which facilitates flowof hydrocarbons through the formation and into the well.

Desirable properties of a hydraulic fracturing fluid include highviscosity, low fluid loss, low friction loss during pumping into thewell, stability under the conditions of use such as in high temperaturedeep wells, and ease of removal from the fracture and well after theoperation is complete. It would be desirable to have a fracturing fluidpossessing all of these properties.

Higher viscosities for the fracturing fluid aids in producing widerfractures. This is particularly advantageous when a viscous solution isused as a pad preceding the acid in combination fracturing-acidizingoperations. More viscous solutions also aid in carrying propping agentsinto the formation when propping agents are used. The common thickeneragents of the prior art such as the natural gums (guar gums, etc.) andstarches require excessive amounts for worthwhile viscosity increases.Furthermore, solutions of said gums and starches are not viscositystable at the higher temperatures encountered in deeper wells, e.g.,above about 200 F.

The fluid loss properties of the fracturing fluid must be low enough topermit build-up and maintenance of the pressures necessary to fracturethe formation. Otherwise, low penetration and/or ineffective fractureswill be obtained. Various fluid loss control agents have been proposedin the past for use with various fracturing fluids. However, at best,the use of such fluid loss control agents is an undesirable complicatingfactor in the preparation and use of fracturing fluids. It would bebetter to have a fracturing fluid which does not require the use of afluid loss control agent.

Low friction loss is desirable so as to avoid excessive well headpressures in pumping the fracturing fluid through the casing and tubingand then into the formation. Otherwise, the frictional losses can becomeprohibitive.

Stability under conditions of use, e.g., retention of sufficientviscosity at temperatures in the order of 200 F. and higher for a periodof time sufficient to carry out the fracturing operation, isparticularly important when the formations penetrated by deep hightemperature wells are being fractured. Fracturing fluids prepared frommany of the prior art thickener materials have little more viscositythan the viscosity of water at temperatures of 200 F., and higher.

The ease of removal of the fracturing fluid from the formation is highlyimportant. One disadvantage of using many highly viscous solutions isthat they are difficult to remove from the pores or the fracture afterthe operation is completed. Other high viscosity solutions sometimesleave a clogging residue in the pores of the formation. This inhibitsproduction and often requires costly clean-up operations after thefracturing operation is completed. It would be desirable to have athickened solution which would break down to a lesser viscosity within ashort time after the fracturing job is complete.

The present invention provides a solution for the above-discussedproblems. The present invention provides methods of fracturing porousformations employing aqueous gels prepared by gelling solutions ofcertain polyacrylamides, and related polymers, as described furtherhereinafter. Said aqueous gels have the above-desirable properties.

Thus, according to the invention, there is provided a method offracturing a subterranean porous formation penetrated by a wellbore,which method comprises injecting down the well and into said formation,at a pressure sufficient to fracture the formation, a fracturing fluidcomprising an aqueous gel, and wherein said gel comprises water to whichthere has been added: a water thickening amount of a water-dispersiblepolymer selected from the group consisting of: polyacrylamides andpolymethacrylamides wherein up to about percent of the carboxamidegroups can be hydrolyzed to carboxyl groups; crosslinked polyacrylamidesand crosslinked polyacrylamides wherein up to about 75 percent of thecarboxamide groups can be hydrolyzed to carboxyl groups; polyacrylicacid and polymethacrylic acid; polyacrylates; polymers of N- substitutedacrylamides wherein the nitrogen atoms in the carboxamide groups canhave from l to 2 alkyl substituents which contain from one to fourcarbon atoms; copolymers of acrylamide with another ethylenicallyunsaturated monomer copolymerizable therewith, sufficient acrylamidebeing present in the monomer mixture to impart said water-dispersibleproperties to the resulting copolymer when it is mixed with water, andwherein up to about 75 percent of the carboxamide groups can behydrolyzed to carboxyl groups; and mixtures of said polymers; a sensibleamount of a watersoluble compound of a polyvalent metal wherein themetal present is capable of being reduced to a lower polyvalent valencestate and which is sufficient to gel said water when the valence of atleast a portion of said metal is reduced to said lower valence state;and an amount of a water-soluble reducing agent which is effective toreduce at least a portion of said metal to said lower valence state.

Thus, one embodiment of the invention comprises using said aqueous gelsas the fracturing fluid. In the practice of the invention, the aqueousgels can be injected down the well and into the porous formationemploying conventional pumping equipment and procedures. if desired, thefracturing fluids used in the practice of the invention can be injectedinto a selected portion or portions of the porous formation. Saidselected portion(s) of the formation can be isolated by employing one ormore well packers at proper locations using packers and methods known inthe art.

The amount of said fracturing fluid injected into the formation willdepend upon the type of formation being treated, the thickness of theformation, the depth or penetration of fracturing desired, etc.Generally speaking, the use of any suitable amount is within the scopeof the invention. Thus, the invention is not limited to the use of anyparticular amount of said aqueous gels as the fracturing fluid. Amountsused in using other fracturing fluids known to the art can be used.Thus, the amount of fracturing fluid can include any amount from 1 to2,000, or more, gallons per vertical foot of formation.

Another embodiment of the invention comprises a combinationfracturing-acidizing treatment. This embodiment' of the invention isparticularly useful where the formation is susceptible to attach by theacid. In this combination method of the invention the aqueous gels ofcellulose ether solutions are used as fracturing pads" and are injected,prior to injection of the acid, at sufficient pressure to create thefracture. The acid is subsequently injected to react with, etch, androughen the fracture faces to provide good conductivity when theoperation is completed.

In combination fracturing-acidizing treatments it is highly desirablethat good penetration of the acid into the formation and good etching ofthe fracture faces be obtained. This is aproblem under almost allcircumstances. The severity of the problem increases as the welltemperature increases because the acid reactivity with the formationincreases with temperature. This results in a reduction in the amount oflive acid penetration. Acid penetration can also be reduced by leak-offat the fracture faces. The acid will naturally react in some of thepores adjacent to the fracture. In extreme cases there may be so-calledworm holing" perpendicular to the fracture face. Another cause of acidleak-off is the presence of natural fractures in the formation beingtreated.

The aqueous gels used in the practice of the invention are particularlywell suited to be used as a fracturing pad in combinationfracturing-acidizing treatments. Said gels serve several purposes. Theyreduce the apparent acid reaction rate by reducing contact rate. Saidaqueous gels will coat the faces of the fracture. Thus, when the aqueousgel pad is displaced by the acid a thin film will remain sufficientlylong to retard the acid reaction rate an amount sufficient to obtaingreater penetration. The acid soon destroys the film gel and performsits intended function of etching and roughening the fracture faces, butnot before its action has been retarded sufficiently to permit a greaterquantity of live acid .to penetrate further into the fracture.

Another valuable purpose of the viscous aqueous gels used in thepractice of the invention is that they increase fracture width andlength. This provides a greater fracture face for the acid to work on,resulting in fractures having greater conductivity. Fracture width isdependent to alarge extent upon the viscosity of the fracturing fluid.As shown in the examples given hereinafter, the aqueous gels used in thepractice of the invention have superior high temperature viscosityproperties when compared to commercially available gels. These superiorviscosity properties result in superior fractures. Still anotheradvantage of the superior viscosity properties is that said gels willcarry more and larger propping agents in those embodiments of theinvention where propping agents are employed.

e.g., sand grains, walnut shell fragments, tampered glass beads,aluminum pellets, and similar materials. Generally speaking, it isdesirable to use propping agents having particle sizes in the range of 8to 40 mesh (U.S. Sieve Series). However, particle sizes outside thisrange can be employed. Propping agents are generally not used in thecombination fracturing-acidizing treatments described herein. However,it is within the scope of the invention to use propping agents in saidcombination treatment. When propping agents are so used they should bemade of materials which are not severely attacked by the acid used.

Acids useful in the practice of the invention include any acid which iseffective in increasing the flow of hydrocarbons through the formationand into the well. Examples of acids which can be used include inorganicacids such as hydrochloric acid, nitric acid, and sulfuric acid; organicacids such as acetic acid, and formic acid; and combinationsof inorganicand organic acids. The concentration or strength of the acid can varydepending upon the type of acid, thetype of formation being treated, andthe results desired in the particular treating operation. For example,when hydrochloric acid solution is being used in a predominantlylimestone formation, the concentration can vary from about 5 to about 38weight percent HCl, with concentrations within the range of 10 to 30weight percent usually preferred. Organic acids are usually used inlower concentrations, e.g., about 10 weight percent. One preferredmixture of inorganic acids and organic acids comprises mixtures ofhydrochloric acid and acetic acid. For example, 15 percent hydrochloricacid solution containing sufficient acetic acid to bring the totalacidity to about 20 to 22 percent, based on equivalent HCl. The acidsused in the practice of the invention can contain any of the knowncorrosion inhibitors, deemulsifying agents, sequestering agents,surfactants, friction reducers, etc., known in the art. The amount ofacid used in any particular instance will depend upon a number offactors including the size or amount of formation to be treated, thetype of formation being treated, the type of acid, the concentration ofthe-acid, and the formation temperature. Thus, the invention is notlimited to using any particular amount of acid in the combinationfracturing-acidizing embodiment of the invention. Any suitable amountfrom about 1 to 750, or more, gallons of acid per vertical foot offormation can be used.

The fracturing operation in accordance with the invention can be carriedout in one or more stages. A stage can comprise the following steps. Ifdesired, depending upon the well conditions, the injection of theaqueous gel can be preceded by a small slug of clean up acid to removescale, wax deposits, etc., and clean the perforations. This clean-upacid injection can be followed with a preflush of water to cool thecasing and the formation. The aqueous gel is then injected. Usually, theacid injection follows the injection of the aqueous gel. The acid slugis then followed with an overflush of water to displace the acid. Thesecond, and

However, it is to be understood the invention is not to be limited tothe above combination of steps. Thus,

in the embodiments of the invention comprising injecting a gelledsolution of a polyacrylamide or a related polymer as the fracturingfluid, the only essential step is the injection of the aqueous gel undersufficient pressure to create the fracture. The injection of the aqueousgel can be preceded by any suitable preflush injection steps, etc., andcan be followed by any suitable subsequent overflush or other clean-upsteps. Similarly, in the combination fracturing-acidizing method of theinvention the only essential steps are the injection of the aqueous geland the subsequent injection of the acid. Generally speaking, in saidcombination treatment it is preferred to inject the acid immediatelyfollowing the injection of the aqueous gel fracturing fluid. However, itis within the scope of the invention to inject a slug of water or otherspacer liquid between the slug of aqueous gel and the slug of acid.

Herein and in the claims, unlessotherwise specified, the term polymer isemployed generically to include both homopolymers and copolymers; andthe term water-dispersible polymers is employed to include thosepolymers which are truly water-soluble and those polymers which aredispersible in water or other aqueous medium to form stable colloidalsuspensions which can be gelled as described herein.

Polymers which can be used to prepare gels for use in the practice ofthe invention include the various polyacrylamides and related polymerswhich are waterdispersible and which can be used in an aqueous medium,with the gelling agents described herein, to give an aqueous gel.Presently preferred polymers include the various substantially linearhomopolymers and copolymers of acrylamide and methacrylamide. Bysubstantially linear it is meant that the polymers are substantiallyfree of crosslinking between the polymer chains. Said polymers can haveup to about 75, preferably up to about 45, percent of the carboxamidegroups hydrolyzed to carboxyl groups. As used herein and in the claims,unless otherwise specified, the term hydrolyzed includes modifiedpolymers wherein the carboxyl groups are in the acid form and also suchpolymers wherein the carboxyl groups are in the salt form, provided saidsalts are water-dispersible. Such salts include the ammonium salts, thealkali metal salts, and others which are water-dispersible. Hydrolysiscan be carried out in any suitable fashion, for example, by heating anaqueous solution of the polymer with a suitable amount of sodiumhydroxide.

Substantially linear polyacrylamides can be prepared by methods known inthe art. For example, the polymerization can be carried out in aqueousmedium, in the presence of a small but effective amount of awater-soluble oxygen-containing catalyst, e.g., a thiosulfate orbisulfate of potassium or sodium or an organic hydroperoxide, at atemperature between about 30 and 80 C. The resulting polymer isrecovered from the aqueous medium, as by drum drying, and can besubsequently ground to the desired particle size. A presently preferredparticle size is such that about 90 weight percent will pass through anumber mesh sieve, and not more than about 10 weight percent will beretained on a 200 mesh sieve (US. Bureau of Standards Sieve Series).

Included among the copolymers which can be used to prepare gels for usein thepractice of the invention are the water-dispersible copolymersresulting from the polymerization of a major proportion of acrylamide ormethacrylamide and a minor proportion of an ethylenically unsaturatedmonomer copolymerizable therewith. It is desirable that sufficientacrylamide or methacrylamide be present in the monomers mixture toimpart to the copolymer the above-described water-dispersibleproperties, for example, from about to 99 percent acrylamide and fromabout 1 to 10 percent other ethylenically unsaturated monomers. Suchother monomers include acrylic acid, methacrylic acid, vinylsulfonicacid, vinylbenzylsulfonic acid, vinylbenzenesulfonic acid, vinylacetate, acrylonitrile, methyl acrylonitrile, vinyl alkyl ether, vinylchloride, maleic anhydride, and the like. Various methods are known inthe art for preparing said copolymers. For example, see U.S. Pat. Nos.2,625,529; 2,740,522; 2,729,557; 2,831,841; and 2,909,508. Saidcopolymers can also be used in the hydrolyzed form, as discussed abovefor the homopolymers.

Polyacrylic acids, including polymethacrylic acid, prepared by methodsknown in the art, can also be used to prepare gels for use in thepractice of the invention.

Polyacrylates, e.g., as described in Kirk-Othmer, Encyclopedia ofChemical Technology, Vol. I, second edition, pages 305 et seq.,lnterscience Publishers, Inc., New York (1963), can also be used toprepare gels for use in the practice of the invention. Examples of saidpolyacrylates include polymers of methyl acrylate, ethyl acrylate,n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutylacrylate, tert-butyl acrylate, n-octyl acrylate, and the like.

Polymers of N-alkyl-substituted acrylamides wherein the nitrogen atomsin the carboxamide groups can have from 1 to 2 alkyl substituents whichcontain one to four carbon atoms can also be used to prepare gels foruse in the practice of the invention. Examples of said N-substitutedacrylamides include, among others, N-methyl acrylamide, N-propylacrylamide, N-butyl acrylamide, N,N-dimethyl acrylamide,N-methyl-N-sec-butyl acrylamide, and the like, at various stages ofhydrolysis, as described above.

Crosslinked polyacrylamides and crosslinked polymethacrylamides, atvarious stages of hydrolysis as described above, can also be used toprepare gels for use in the practice of the invention. In general, saidcrosslinked polyacrylamides can be prepared by the methods describedabove, but including in the monomeric mixture a suitable amount of asuitable cross-linking agent. Examples of crosslinking agents includemethylenebisacrylamide, divinylbenzene, vinyl ether, divinyl ether, andthe like. Said crosslinking agents can be used in small amounts, e.g.,up to about 1 percent by weight of the monomeric mixture. Suchcrosslinking is to be distinguished from any crosslinking which occurswhen solutions of polymers are gelled as described herein.

ln preparing aqueous gels for use in the practice of the invention, itis desirable for economic and other reasons to use water which isreadily available in the field. Frequently, the only readily availablewater is field brine, produced from wells in the field, and containinglarge amounts of total dissolved solids. As discussed furtherhereinafter, the amount of dissolved solids contained in such brinesaffects the gelling rate and the life span or stability of the gel. Insome instances it has been impossible to obtain gels when using strongbrines. it has been discovered that for best results when using brinescontaining large amounts of total dissolved solids, the polyacrylamideor related polymer which is used should be one wherein not more thanabout 14, preferably not more than about 12, percent of the carboxamidegroups have been hydrolyzed to carboxyl groups. Such polymers comprise asubgroup of polymers for use in the practice of the invention.

Said subgroup of polymers includes: polyacrylamides andpolymethacrylamides wherein from 0.1 to about 14 percent of thecarboxamide groups are hydrolyzed to carboxyl groups; crosslinkedpolyacrylamides and crosslinked polyacrylamides wherein from 0.1 toabout 14 percent of the carboxamide groups are hydrolyzed to carboxylgroups; copolymers of acrylamide with another ethylenically unsaturatedmonomer copolymerizable therewith, sufficient acrylamide being presentin the monomer mixture to impart said waterdispersible properties to theresulting copolymer when it is mixed with water, and wherein from 0.1 toabout 14 percent of the carboxamide groups are hydrolyzed to carboxylgroups; and mixtures of said polymers.

All the polymers useful in preparing gels for use in the practice of theinvention are characterized by high molecular weight. The molecularweight is not critical so long as the polymer has the above-describedwaterdispersible properties. it is preferred that the polymer have amolecular weight of at least 100,000. The upper limit of molecularweight is unimportant so long as the polymer is water-dispersible, andthe aqueous gel prepared therefrom can be pumped. Thus, polymers havingmolecular weights as high as 20,000,000 or higher, and meeting saidconditions, can be used.

The amount of the above-described polymers used in preparing gels foruse in the practice of the invention can vary widely depending upon theparticular polymer used, the purity of said polymer, and propertiesdesired in said aqueous gels. In general, the amount of polymer usedwill be a water-thickening amount, i.e., at least an amount which willsignificantly thicken the water to which it is added. For example,amounts in the order of 25 to 100 parts per million weight (0.0025 to0.01 weight percent) have been found to significantly thicken water.Distilled water containing 25 ppm of a polyacrylamide having a molecularweight of about 10 X l has a viscosity increase of about 41 percent. At50 ppm the viscosity increase is about 106 percent. At 100 ppm theviscosity increase is about 347 percent. As another example, distilledwater containing 25 ppm of a polyacrylamide having a molecular weight ofabout 3.5 X has a viscosity increase of about 23 percent. At 50 ppm theviscosity increase is about 82 percent. At 100 ppm the viscosityincrease is about 24] percent. Generally speaking, amounts of theabove-described polymers in the range of from 0.0025 to 5, preferablyfrom 0.01 to 1.5, more preferably 0.025 to 0.4, weight percent, based onthe weight of water, can be used in preparing gels for use in thepractice of the invention. However, amounts outside said ranges can beused. in general, with the proper amounts of polyvalent metal andreducing agent, the amount of polymer used will determine theconsistency of the gel obtained. Small amounts of polymer will usuallyproduce liquid mobile gels which can be readily pumped. Large amounts ofpolymer will usually produce thicker, more viscous, somewhat elasticgels. If desired, said thick gels can be thinned by dilution with waterto any desired concentration of polymer. This can be done by mechanicalmeans, e.g., stirring, pumping, or by means of a suitable turbulenceinducing device to cause shearing, such as a jet nozzle. Thus, there isreally no fixed upper limit on the amount of polymer which can be used.

However, it has been discovered that when a liquid mobile gel isdesired, it is definitely preferred to first prepare a concentrated geland dilute the more concentrated gels before they become too viscous. Ingeneral, dilute gels are more difficult to prepare in that, for onething, gelling times are longer. Another advantage is that, in general,less gelling agents are required for a given viscosity.

Metal compounds which can be used in preparing gels for use in thepractice of the invention are watersoluble compounds of polyvalentmetals wherein the metal is present in a valence state which is capableof being reduced to a lower polyvalent valence state. Examples of suchcompounds include potassium permanganate, sodium permanganate, ammoniumchromate, ammonium dichromate, the alkali metal chromates, the alkalimetal dichromates, and chromium trioxide. Sodium dichromate andpotassium dichromate, because of low cost and ready availability, arethe presently preferred metal-containing compounds. The hexavalentchromium in said chromium compounds is reduced in situ to trivalentchromium by suitable reducing agents, as discussed hereinafter. In thepermanganate compounds the manganese is reduced from +7 valence to +4valance as in MnO The amount of said metal-containing compounds usedwill be a sensible amount, i.e., a small but finite amount which is morethan incidental impurities, but which is effective or sufficient tocause subsequent gellation when the metal in the polyvalent metalcompound is reduced to a lower polyvalent valence state. The lower limitof the concentration of the starting metal-containing compound willdepend upon several factors including the particular type of polymerused, the concentration of the polymer in the water to be gelled, thewater which is used, and the type of gel product desired. For similarreasons, the upper limit on the concentration of the startingmetal-containing compound also cannot always to precisely defined.However, it should be noted that excessive amounts of the starting metalcompound, for example +6 chromium, which can lead to excessive amountsof +3 chromium when there is sufficient reducing agent present to reducethe excess +6 chromium, can adversely affect the stability of the gelsproduced. As discussed further hereinafter, this provides one valuablemethod for controlling gel stability or life span so as to obtain gelswhich will break down with time and/or temperature to permit ready wellclean-up subsequent to a fracturing operation. As a general guide, theamount of the start ing polyvalent metal-containing compound used inpreparing aqueous gels for use in the practice of the invention will bein the range of from 0.05 to 60,

preferably 0.5 to 40, weight percent of the amount of the polymer used.Stated another way, the amount of the starting polyvalentmetal-containing compound used will usually be an amount sufficient toprovide at least about 3 X preferably at least 3 X 10' gram atoms ofsaid metal capable of being reduced per gram of polymer. Preferably, theamount of said metal capable of being reduced which is used will notexceed 4 X 10 more preferably 2 X 10 gram atoms of said metal per gramof polymer. However, in some situations it may be desirable to useamounts of the starting polyvalent metal-containing compound which areoutside the above ranges. Such use is within the scope of the invention.Those skilled in the art can determine the amount of starting polyvalentmetal-containing compound to be used by simple experiments carried outin the light of this disclosure. For example, when brines, such as arecommonly available in producing oil fields, are used in the water inpreparing gels for use in the practice of the invention, less of thestarting polyvalent metal-containing compound is required than whendistilled water is used. Suitable gels can be prepared using brineshaving a wide range of dissolved solids content, depending upon theparticular polymer and brine used. Gellation rates are frequently fasterwhen using said brines. Such oil field brines commonly contain varyingamounts of sodium chloride, calcium chloride, magnesium chloride, etc.Sodium chloride is usually present in the greatest concentration.

Generally speaking, water having a low (or essentially none) totaldissolved solids content is the preferred medium for preparing the gelsdescribed herein. Generally speaking, when brines are used suitable gelscan be obtained when the total dissolved solids content is not greaterthan about 60,000, and the amount of polyvalent metal ions such ascalcium, magnesium, etc., is not greater than about 6,000.

However, when using polyacrylamides and related polymers having not morethan about 14 percent of the carboxamide groups hydrolyzed to carboxylgroups, water having a total dissolved solids content greater than60,000 ppm by weight can be used for preparing the gels used in thepresent invention. Good results can be obtained with such polymers whenusing brines having a total dissolved solids content much greater thanabout 60,000 ppm by weight, e.g., up to at least about 174,000 ppm byweight. Furthermore, of said total dissolved solids, the amount ofpolyvalent metal ions such as calcium, magnesium, etc., can be greaterthan 6,000 ppm by weight. Good results can be obtained when using brineshaving greater than 12,000 ppm by weight of said polyvalent metal ions.

Another advantage in using strong field produced brines in preparing theaqueous gels described herein, in addition to the economic advantage ofusing readily available materials, is that the problem of disposing ofsuch brines is lessened.

The word water" is used generically herein and in the claims, unlessotherwise specified, to include the above-described brines, fresh water,and other aqueous media which can be gelled in accordance with theinvention.

Suitable reducing agents which can be used in the practice of theinvention include sulfur-containing compounds such as sodium sulfite,sodium hydrosulfite, sodium metabisulfite, potassium sulfite, sodiumbisulfite, potassium metabisulfite, sodium sulfide, sodium thiosulfate,ferrous sulfate, thioacetamide,

and others; and nonsulfur-containing compounds such as hydroquinone,ferrous chloride, p-hydrazinobenzoic acid, hydrazine phosphite,hydrazine dichloride, and others. Some of the above reducing agents actmore quickly than others, for example, sodium thiosulfate usually reactsslowly in the absence of heat, e.g., heating to about l 30 F. Thepresently most preferred reducing agents are sodium hydrosulfite orpotassium hydrosulfite, and sodium or potassium thiosulfate.

The amount of reducing agent to be used in preparing the gels used inthe practice of the invention will be a sensible amount, i.e., a smallbut finite amount which is more than incidental impurities, but which iseffective or sufficient to reduce at least a portion of the highervalence metal in the starting polyvalent metalcontaining compound to alower polyvalent valence state. Thus, the amount of reducing agent to beused depends, to some extent at least, upon the amount of the startingpolyvalent metal'containing compound which is used. In many instances,it will be preferred to use an excess of reducing agent to compensatefor dissolved oxygen in the water, exposure to air during preparation ofthe gels, and possible contact with other oxidizing substances such asmight be encountered in field operations. As a general guide, the amountof reducing agent used will generally be within the range of from 0.1 toat least 150, preferably at least about 200, weight percent of thestoichiometric amount required to reduce the metal in the startingpolyvalent metal compound to said lower polyvalent valence state, e.g.,+6 Cr to +3 Cr. However, in some instances, it may be desirable to useamounts of reducing agent outside said ranges. The use of such amountsis within the scope of the invention. Those skilled in the art candetermine the amount of reducing agent to be used by simple experimentscarried out in the light of this disclosure.

Various methods can be used for preparing the aqueous gels used in thepractice of the invention. Either the metal-containing compound or thereducing agent can be first added to a solution or dispersion of thepolymer in water or other aqueous medium, or said metal-containingcompound and said reducing agent can be added simultaneously to thesolution or aqueous medium containing the polymer. Generally speaking,where convenient, the preferred method is to first disperse the polymerin the water or other aqueous medium. The metal-containing compound isthen added to the solution or aqueous medium containing the polymer andthe reducing agent, with stirring. The reducing agent is then added tothe dispersion of polymer, with stirring. Gellation starts as soon asreduction of some of the higher valence metal in the starting polyvalentmetal containing compound to a lower valence state occurs. Thenewly-formed lower valence metal ions, for example +3 chromium obtainedfrom +6 chromium, effect rapid crosslinking of the polymer and gellationof the solution or aqueous medium containing same.

it is also within the scope of the invention to prepare a dry mixture ofthe polymer, the metalcontaining compound and the reducing agent, inproper proportions, and then add this dry mixture to the proper amountof water.

An advantage of the invention is that ordinary ambient temperatures andother conditions can be used in practically all instances in preparingthe aqueous gels used in the practice of the invention or aqueousmediums containing same. However, in some instances, a small amount ofheat may be desirable to aid in the formation of the gel, e.g., heatingto a temperature of about l25130 F.

Aqueous gels used in the practice of the invention can be preparedhaving a wide range of viscosities or firmness ranging from lowviscosity or highly mobile gels having a relatively low viscosity up tothick, viscous, somewhat elastic gels which are relatively nonmobile.The choice of gel viscosity or concentration will depend upon the use tobe made of the gel. The actual viscosity and/or gel strength of the gelwill depend upon the type and concentration of the polymer, the type andamount of starting polyvalent metal compound used, and the type andamount of reducing agent used.

One procedure which can be used to prepare said gels is to prepare arelatively concentrated or high viscosity gel and dilute same to aviscosity or concentration suited for the actual use of the gel. In manyinstances, this procedure results in a more stable gel, in addition tothe advantages mentioned above. This should be taken into considerationsince in the practice of the present invention highly stable gels arenot, generally speaking, desirable. Gels having good initial stabilitysufficient to permit pumping and placement in the formation to fracturesame, but which will break down with time and/or temperature to permiteasy well clean-up are most useful in the practice of the presentinvention. Generally speaking, it is preferred that said gels have astability, e.g., viscous life, within the range of about 15 minutes toabout 12 hours.

When employing said dilution technique a starting solution or dispersionof polymer containing, for example, 1,000 to 10,000 ppm (0.1 to 1 wt.percent), or more, of polymer can be used. This solution or dispersionis then gelled by the addition of suitable amounts of polyvalent metalcompound and reducing agent. After gellation has proceeded to thedesired extent, the resulting gel can be diluted with water to theconcentration or viscosity most suited for its intended use. The moreconcentrated polymer solutions or dispersions usually have a faster rateof gellation. Thus, in most instances, it will be preferred to carry outthe dilution soon after the components of the gel have been added to thewater or other aqueous medium, e.g., within about 5 to 30 minutes.Preferably, the concentration of the polymer in the concentrated gel"will be at least twice that in the final gel. Dilution of the gelretards the rate of gellation. Thus, this dilution technique can beemployed to control the gellation rate, if desired. One advantage ofsaid dilution technique is that it is usually more convenient to weighout and handle the larger quantities of reagents.

Generally speaking, the pH of the final solution 'of the gellingreagents is preferably less than 7, more preferably in the order of 6.In general, pH is not controlling, but higher pH values retard gellationrate. In general, the pH of the gelling solution will depend upon thereducing agent used. If desired, the pH can be adjusted by the additionof a suitable acid, depending upon the reducing agent used.

Herein and in the claims, unless otherwise specified, the aqueous gelsused in the practice of the invention are defined for convenience, andnot by way of limitation, in terms of the amount of polymer containedtherein, irrespective of whether or not all the polymer has entered intothe gel-forming reaction. For example, a 1 weight percent or 10,000 ppmgel is a gel which was prepared from a starting polymer solution ordispersion which contained 1 weight percent of 10,000 ppm by weight ofpolymer. The same system is employed for the gels prepared by theabove-described dilution technique.

The following examples will serve to further illustrate the invention.

EXAMPLE 1 A series of runs was made to illustrate the formation ofaqueous gels. For these runs a stock solution was prepared by dissolving6 grams of a substantially linear polyacrylamide in 2 liters ofBartlesville tap water. Said polyacrylamide was a commercially availablematerial having a molecular weight of about 10 X 10, a nitrogen contentof about 12 weight percent, and was about 21 percent hydrolyzed. Theresulting solution contained about 3,000 ppm of said polyacrylamide. Toindividual 200 ml portions of this stock solution there were addedvarying amounts of a l0 weight percent solution of Na S Oin distilledwater, with stirring for 10 seconds; and then varying amounts of a 10weight percent solution of Na Cr O -2H O in distilled water, withstirring for 10 minutes. Gels were formed in each instance. The resultsof these test runs are set forth in Table 1 below.

TABLE I Na Cr,O '2H O Na S O Apparent Viscosities,

No. grams grams 1 m inutc 48 hours 1 0 28 30 2 0.025 0.025 38 37 3 0.050.05 49 36 4 0.05 0.10 35 56 5 0.10 0.10 41 47.5 6 0.10 0.15 34 53 70.15 0.15 28 50 8 0.25 0.25 42 (1) Model 35, Fann VG meter (300 rpm) Theabove data indicate that with increasing amounts of Na Cr O '2H O, withsufficient reducing agent present to reduce Cr to Cr, the rate ofgellation increases. The data also indicate that for a given amount ofNa Cr O -2H O, as the amount of reducing agent present increases, theoverall rate of gellation increases.

The gels obtained in the above runs were stable gels at ordinaryconditions of temperature. However, in many instances such as when usedin deep high temperature wells, e.g., formation temperatures greaterthan about 200 R, such gels are useful in the practice of the presentinvention because they will break down after exposure to suchtemperatures for a given period of time.

EXAMPLE 11 A series of aqueous gels was prepared using variouscommercially available polyacrylamides having different molecularweights and different amounts of the carboxamide groups thereinhydrolyzed to carboxylic groups. Said gels were prepared using a typicalsample of brine produced in the East Hull Silk Field in Archer County,Texas. This brine analyzed as follows:

ppm by weight NaCl 124,000 CaCl 34,200 MgCl- ,-2H,O 16,000 174,200

Each of the aqueous gels tested contained 2,000 ppm by weight of polymerand was prepared as follows. Two grams of each polymer were added to 1liter of said brine. To each of the resulting solutions there was added,with stirring, an amount of a 10 weight percent solution of sodiumhydro-sulfite sufficient to provide 300 ppm by weight of Na S O Therewas then added, with stirring, an amount ofa 10 weight percent solutionof sodium diehromate sufficient to provide 300 ppm of Na Cr O7 ZH O.Properties of each of the resulting gels, or solutions, are set forth inTable ll below.

TABLE II Polymer PF l 160 gel PF 1110 B X 10 l00,000 clear solution;

gel not as smooth as PF 1 I60 gel clear solution, but did not form gelsolution precipitated;

gelled, but gel broke in 24 hrs. solution turbid, gelled, but gel brokein 24 hrs. clear solution, but did not form gel solution badlyprecipitated;

did not form gel WC 500 3.5 X l 4.4 2.4

DP 1000 I0 X10 21 5.9 2.3

WC 773 l6 X 10 35 3.3 2.5

PF 1130 10.5 X 10 40 2.7

Additional test runs were made on other polyacrylamide solutions, gelledand ungelled, containing 1,500, 3,000, and 10,000 ppm by weight ofpolyacrylamides having molecular weights and percent hydrolyzed valuescomparable to those set forth in Table 11 above. These runs were madeusing a synthetic brine containing approximately 86,000 ppm by weighttotal dissolved solids. The results obtained were essentially the sameas set forth in Table I.

The gels from Runs 1 and 2 in the above Table II were good stable gelsand are well suited for injection into nonfractured porous formation forwater diversion purposes. However, such gels would also be useful asfracturing fluids in deep high temperature wells, as described above,because they would break down upon exposure to the high temperaturesthere existing for a given period of time. The polymers used in Runs 3,6, and 7 did not form gels. It would be necessary to use a watercontaining less dissolved solids in order to obtain gels from thesepolymers. Gels have been made from the polymers used in said Runs 3, 6,and 7 when using fresh water. The gels in Runs 4 and 5 broke down within24 hours at room temperature to a viscosity approaching that of water.These gels would be useful in the practice of the present invention.

EXAMPLE Ill An aqueous gel was prepared using the PF 1160 polyacrylamideof Run 1 in Example 11, and a strong field brine containing about100,000 ppm total dis solved solids. The gel contained 3,000 ppm byweight of said polymer. Gellation was effected by the addition of 1,000ppm of Na Cr O -2H O and l,000 ppm of Na S O. Two hours after additionof said gelling agents, the viscosity of the gel was too great tomeasure on a Brookfield viscometer.

The gel was allowed to stand at room temperature for about 2 weeks.There was no apparent change. A sample of the gel was placed in a glasscontainer and then heated in a steel bomb for 18 hours at 360F. Aftercooling to room temperature, inspection showed the gel had completelybroken down. The fluid remaining in the glass container exhibited aviscosity which visually was like that of water.

The 360 F. temperature in the above test is comparable to the 350 F.formation temperatures in deep Ellenburger gas wells in the Gomez field,Pecos Coun ty, Texas.

There are several method by which gelled solutions of polyacrylamidesand related polymers (as defined herein) can be caused to break downwith time so that their final viscosity approximates that for ungelledpolymer solution, or water. One method is to use excessive amounts ofthe gelling agents which will produce adequate viscosity and gel liferequired during the fracture treatment, but which will subsequentlycause breakdown to a thin solution having a viscosity approaching oressentially the same as water, allowing rapid well clean-up following afracture treatment.

As discussed elsewhere herein, another method of causing gelledsolutions of polyacrylamides and related polymers to break down toviscosities approaching that of water is to expose said gels to elevatedtemperatures. Said gels, even gels which are normally stable for longperiods of time at ordinary temperatures, are selfbreaking at elevatedtemperatures, and the breaking time decreases with increases intemperature and increased time of exposure to said elevatedtemperatures.

Still another method for causing said gels to break down with time toviscosities approaching the viscosity of water is to use brines, e.g.,water containing increased amounts of dissolved solids, in preparing thegels. Significantly less gelling agents are required to gel solutions ofpolymers made with brines than are required to gel solutions made withfresh water. As the salinity of the water increases, the rate ofgellation increases. Also, for a given amount of polymer and givenamounts of gelling agents, the stability or life span of the geldecreases with increasing salinity of the water.

Thus, the rates of gellation and solutions of polyacrylamides andrelated polymers (as defined herein), and/or the life span of theresulting gel, can be tailored in accordance with conditions encounteredin the field. This can be done by taking into consideration thetemperature of the formation where the gel is to be used, the amount ofgelling agents used in preparing the gels, the water used in preparingthe gels, and the particular polymer used. A gel can be tailored to havea life span of 18, 12, 8, 4, 2 hours, or less, so that the gel willbreak down to a viscosity approaching that of water within the timeselected. This will permit ready well clean-up after the fracturingtreatment and permit ready removal of gel residue, such as by producingof formation fluids. Gels can be prepared which will break back toviscosities of less than about centipoises, or even to the viscosity ofwater, by proper consideration of the above-mentioned factors.

One presently preferred method of carrying out a .fracturing operationin accordance with the present invention comprises preparing a basefracturing fluid comprising a solution of a polymer (as defined herein),adding to this base fluid (a) a polyvalent metal compound such as sodiumdichromate or (b) a reducing agent such as sodium hydrosulfite or sodiumthiosulfate, pumping a slug of said base fracturing fluid down the welland into the formation under sufficient pressure to create the fracture,and during said pumping adding to said base fracturing fluid the otherof said reagents (a) and (b) which was not previously added thereto. Itis also within the scope of the invention to incorporate all thecomponents of the aqueous gel into a stream of water while it is beingpumped, e.g., into a well. For example, polyacrylamide could be addedfirst to the flowing stream of water and the other components addedsubsequently in any suitable order. Turbulent flow conditions in thepipe will provide proper mixing and gellation will occur during saidpumping.

In the practice of the invention, the fracturing fluids can be injectedinto the formation at any suitable pressures sufficient to overcome theweight of the overburden. As will be understood by those skilled in theart,

this will vary from region to region. However, generally speaking, saidpressures will be in the range of from 0.5 to 1.5 psi per foot of welldepth.

While certain embodiments of the invention have been described forillustrative purposes, the invention is not limited thereto. Variousother modifications or embodiments of the invention will be apparent tothose skilled in the art in view of this disclosure. Such modificationsor embodiments are within the spirit and scope of the disclosure.

I claim:

l. A method of fracturing a subterranean porous formation penetrated bya wellbore, which method comprises injecting down the well and into saidformation, at a pressure sufficient to fracture the formation, afracturing fluid comprising an aqueous gel, and wherein said gelcomprises water to which there has been added:

a water-thickening amount of a water-dispersible polymer selected fromthe group consisting of: polyacrylamides and polymethacrylamides whereinup to about 75 percent of the carboxamide groups can be hydrolyzed tocarboxyl groups; crosslinked polyacrylamides and crosslinkedpolyacrylamides wherein up to about 75 percent of the carboxamide groupscan be hydrolyzed to carboxyl groups; polyacrylic acid andpolymethacrylic acid; polyacrylates; polymers of N-substitutedacrylamides wherein the nitrogen atoms in the carboxamide groups canhave from 1 to 2 alkyl substituents which contain from one to fourcarbon atoms; copolymers of acrylamide with another ethylenicallyunsaturated monomer copolymerizable therewith, sufficient acrylamidebeing present in the monomer mixture to impart said waterdispersibleproperties to the resulting copolymer when it is mixed with water, andwherein up to about 75 percent of the carboxamide groups can behydrolyzed to carboxyl groups; and mixture of said polymers;

a sensible amount of a water-soluble compound of a polyvalent metalwherein the metal present is capable of being reduced to a lowerpolyvalent valence state and which is sufficient to gel said water whenthe valence of at least a portion of said metal is reduced to said lowervalence state; and

an amount of a water-soluble reducing agent which is effective to reduceat least a portion of said metal to said lower valence state.

2. A method according to claim 1 wherein said aqueous gel compriseswater to which there has been added: from 0.0025 to 5 percent of saidpolymer, based upon the weight of said water;

from 0.05 to weight percent of said polyvalent metal compound based uponthe weight of said polymer;

from 0.1 to at least about 200 percent of the stoichiometric amount ofsaid reducing agent required to reduce said polyvalent metal to saidlower polyvalent valence state; and

wherein the initial total dissolved solids content of said water is notgreater than about 60,000 ppm by weight, and of said dissolved solids,the amount of polyvalent metal ions present is not greater than about6,000 ppm by weight.

3. A method according to claim 2 wherein said total dissolved solidscontent of said water is not greater than about 40,000 ppm by weight,and the amount of said polyvalent metal ions is not greater than about3,000 ppm by weight.

4. A method according to claim 2 wherein said polymer is a substantiallylinear polymer of acrylamide.

5. A method according to claim l wherein said compound of a polyvalentmetal is a compound of chromium wherein the valence of the chromium is+6 and the valence of at least a portion of said chromium is reduced to+3.

6. A method according to claim 5 wherein said chromium compound isselected from the group consisting of ammonium chromate, ammoniumdichromate, the alkali metal chromates and dichromates, chromiumtrioxide, and mixtures thereof.

7. A method according to claim 6 wherein said reducing agent is selectedfrom the group consisting of hydroquinone, sodium sulfide, sodiumhydrosulfite, sodium metabisulfite, potassium sulfite, sodium bisulfite,potassium metabisulfite, sodium sulfite, sodium thiosulfate, ferroussulfate, ferrous chloride, phydrazinobenzoic acid, hydrazine phosphite,hydrazine dihydrochloride, and mixtures thereof.

8. A method according to claim 7 wherein:

said chromium compound is sodium dichromate or potassium dichromate; and

said reducing agent is sodium hydrosulfite, potassium hydrosulfite,sodium thiosulfate, or potassium thiosulfate.

9, A method according to claim 2 wherein:

said polymer is a substantially linear polyacrylamide;

said polyvalent metal compound is sodium dichromate; and

said reducing agent is sodium hydrosulfite or sodium thiosulfate. 10. Amethod according to claim it wherein: said polymer is selected from thegroup consisting of: polyacrylamides and polymethacrylamides whereinfrom 0.1 to about 14 percent of the carboxamide groups are hydrolyzed tocarboxyl groups; crosslinked polyacrylamides and crosslinkedpolyacrylamides wherein from 0.1 to about 14 percent of the carboxamidegroups are hydrolyzed to carboxyl groups; copolymers of acrylamide withanother ethylenically unsaturated monomer copolymerizable therewith,sufficient acrylamide being present in the monomer mixture to impartsaid water-dispersible properties to the resulting copolymer when it ismixed with water, and wherein from 0.1 to about 14 percent of thecarboxamide groups are hydrolyzed to carboxyl groups; and mixtures ofsaid polymers; and

wherein the initial total dissolved solids content of said water isgreater than about 60,000 ppm by weight.

llll. A method according to claim wherein said aqueous gel compriseswater to which there has been added:

from 0.0025 to 5 weight percent of said polymer,

based upon the weight of said water;

from 0.05 to 60 weight percent of said polyvalent metal compound basedon the weight of said polymer; and

from 0.1 to at least about 200 percent of the stoichiometric amount ofsaid reducing agent required to reduce said polyvalent metal to saidlower polyvalent valence state.

12. A method according to claim M wherein said polymer is asubstantially linear polymer of acrylamide.

lid

13. A method according to claim 12 wherein said compound of a polyvalentmetal is a compound of chromium wherein the valence of the chromium is+6 and the valence of at least a portion of said chromium is reduced to+3.

lid. A method according to claim 113 wherein:

said chromium compound is selected from the group consisting of ammoniumchromate, ammonium dichromate, the alkali metal chromates anddichromates, chromium trioxide, and mixtures thereof; and

said reducing agent is selected from the group consisting ofhydroquinone, sodium sulfide, sodium hydrosulfite, sodium metabisulfite,potassium sulfite, sodium bisulfite, potassium metabisulfite, sodiumsulfite, sodium thiosulfate, ferrous sulfate, ferrous chloride,p'hydrazinobenzoic acid, hydrazine phosphite, hydrazine dihydrochloride,and mixtures thereof.

15. A method according to claim it) wherein:

said polymer is a substantially linear polyacrylamidc;

said polyvalent metal compound is sodium dichromate; and

said reducing agent is sodium hydrosulfite or sodium thiosulfate.

lltS. A method according to claim ll wherein:

said formation is susceptible to attack by an acid;

a slug of a fracturing fluid comprising said gel is injected into saidformation; and

a slug of an acid is injected into said formation subsequent to theinjection of said fracturing fluid.

ll7. A method according to claim 16 wherein a slug of a spacer fluid isinjected into said formation after the injection of said fracturingfluid and prior to injecting said acid.

M5. A method according to claim 17 wherein said acid is selected fromthe group consisting of hydrochloric acid, formic acid, acetic acid, andmixtures thereof.

19. A method according to claim llg wherein said aqueous gel compriseswater to which there has been added:

from 0.01 to 1.5 weight percent of said polymer,

based upon the weight of said water;

from 0.5 to 40 weight percent of said polyvalent metal compound, basedupon the weight of said polymenand from 0.1 to at least about percent ofthe stoichiometric amount of said reducing agent required to reduce saidpolyvalent metal to said lower polyvalent valence state.

20. A method according to claim 19 wherein:

said polymer is a substantially linear polyacrylamide;

said polyvalent metal compound is an alkali metal dichromate', and

said reducing agent is sodium hydrosulfite or sodium thiosulfate.

Ell. A method according to claim 20 wherein:

said polyvalent metal compound is sodium dichromate; and

said acid comprises a solution of hydrochloric acid.

22. A method according to claim 20 wherein:

said polymer is a substantially linear polyacrylamide wherein from 0.1to about 14 percent of the car boxamide groups are hydrolyzed tocarboxyl groups;

said polyvalent metal compound is sodium dichromate; and

said acid comprises a solution of hydrochloric acid.

23. A method according to claim 1 wherein the temperature of saidformation is greater than about 200 F. and the life span of said gel issuch that it breaks down to a viscosity approaching that of water inless than about 18 hours.

24. A method according to claim 1 wherein an excessive amount of saidgelling agents is used relative to the amount of said polymer, so thatthe life span of said gel is such that it breaks down to a viscosityapproaching that of water in less than about 18 hours.

25. A method of fracturing a subterranean formation penetrated by awellbore, which method comprises, in combination, the steps of:

A. forming a base fracturing fluid by adding to a given quantity ofwater from 0.01 to 1.5 weight percent, based on the weight of saidwater, of a polymer selected from the group consisting of:polyacrylamides and polymethacrylamides wherein up to about 75 percentof the carboxamide groups can be hydrolyzed to carboxyl groups;crosslinked polyacrylamides and crosslinked polyacrylamides wherein upto about 75 percent of the carboxamide groups can be hydrolyzed tocarboxyl groups; polyacrylic acid and polymethacrylic acid;polyacrylates; polymers of N-substituted acrylamides wherein thenitrogen atoms in the carboxamide groups can have from 1 to 2 alkylsubstituents which contain from one to four carbon atoms; copolymers ofacrylamide with another ethylenically unsaturated monomercopolymerizable therewith; sufficient acrylamide being present in themonomer mixture to impart said waterdispersible properties to theresulting copolymer when it is mixed with water, and wherein up to about75 percent of the carboxamide groups can be hydrolyzed to carboxylgroups; and mixtures of said polymers;

B. adding to said base fracturing fluid one of (a) from 0.5 to 40 weightpercent of a water-soluble compound of chromium wherein the valence ofthe chromium is +6 and which is sufficient to gel said water when thevalence of at least a portion of said chromium is reduced from +6 to +3,or (b) from 0.5 to at least about 150 percent of the stoichiometricamount ofa water-soluble reducing agent which is effective to reduce thevalence of said chromium from +6 to +3;

C. pumping a slug of said base fracturing fluid of step (B) down saidwell and into said formation under a pressure sufficient to create afracture in said formation; and

D. during said pumping adding to said base fracturing fluid the other ofsaid reagents (a) and (b) which was not added thereto in said step (B).

26. A method according to claim 25 wherein:

said polymer is a substantially linear polyacrylamide;

said chromium compound is sodium dichromate or potassium dichromate;

said reducing agent is sodium hydrosulfite or sodium thiosulfate; and

wherein said water has an initial total dissolved solids content notgreater than about 60,000 ppm by weight, and of said dissolved solids,the amount of polyvalent metal ions present is not greater than about6,000 ppm by weight.

27 A methodaccording to claim 25 wherein: said polymer is asubstantially linear polyacrylamide formation is susceptible to attachby an acid, and said method comprises in further combination, the stepof:

E. injecting a slug of an acid into said formation subsequent to theinjection of said fracturing fluid as in said steps (C) and (D).

29. A method according to claim 28 wherein a slug of a spacer fluid isinjected into said formation after 30 said steps (C) and (D) and priorto said step (E).

30. A method according to claim 28 wherein:

said polymer is a substantially linear polyacrylamide;

said chromium compound is sodium dichromate or potassium dichromate;

said reducing agent is sodium hydrosulfite or sodium thiosulfate;

said acid comprises a solution of hydrochloric acid;

and

wherein said water has an initial total dissolved solids content notgreater than about 60,000 ppm by weight, and of said dissolved solids,the amount of polyvalent metal ions present is not greater than about60,000 ppm by weight, and of said dissolved solids, the amount ofpolyvalent metal ions present is not greater than about 6,000 ppm byweight.

31. A method according to claim 28 wherein:

said polymer is a substantially linear polyacrylamide;

said chromium compound is sodium dichromate or potassium dichromate;

said reducing agent is sodium hydrosulfite or sodium thiosulfate;

said acid comprises a solution of hydrochloric acid;

and

wherein the initial total dissolved solids content of said water isgreater than about 60,000 ppm by weight.

UNITW STATES PATH??? @FFIGE CERTIFICATE OF CGCTION moo 3 727 689 DamonApril 17, 1973 Richard L, Clampitt It is certified that error appears inthe above iclentified patent and that said Patent are hereby correctedas shown below:

Column 2 line 25, change "polyacrgrlanddes" to polymethacr'ylamidescolumn 6 line 1 ,6 change above to herein column '7, line 15, change .rrlamides" to polymethacrylamides column 16, line 20, change rylamides topolymethacrylamides column 16, line 35, change "ntbcture" to 3 mixturescolumn 17, line 40, change polyacrylamides to ymethacrylamides column17, line 60, change on to upon and 11.9 line 23 change "polyacrylamides"to polymethacrylamides Signed and sealed this 5th day of February 1974-.

[SEAL] Attest:

EDWARD M. FLETCHER,JR. RENE D TEGTMEYER Arresting Officer ActingCommissioner of Patents UNITED STATES PATBIT OFFICE CERTIFICATE OFCORRECTION Patent No, 3,727,689 Dated= A ril 17,1973

Richard L. Clampitt It is certified that error appears in theabove-identified patent and that sa Lettens Patent are hereby correctedas shown below:

Column 2, line .25, change "polyacrylamid'es" to polymethacrylamidescolumn 6, line he, change "above" to herein ---5 column '7, line 15,change pcftyecmiemides" to polymethacrylamides column 16, line 20,change po.1 :y'e,crf ylam sies to polymethacrylamides column 16, line35, change "mixb to m mixtures column 17, line 1+0, changepolyz-lcrylamides" to um polymethacrylamides colwm 17, line 60, change"on to upon and 19, line 23, change "polyacrylamides" topolymethaczwlamides Signed and sealed this 5th day of February 1974.

(SEAL) Attest:

EDWARD M.FLETCHER,IJR. RENE D. TEGTMEYER Attesting Officer ActingColmnissioner of Patents

2. A method according to claim 1 wherein said aqueous gel compriseswater to which there has been added: from 0.0025 to 5 percent of saidpolymer, based upon the weight of said water; from 0.05 to 60 weightpercent of said polyvalent metal compound based upon the weight of saidpolymer; from 0.1 to at least about 200 percent of the stoichiometricamount of said reducing agent required to reduce said polyvalent metalto said lower polyvalent valence state; and wherein the initial totaldissolved solids content of said water is not greater than about 60,000ppm by weight, and of said dissolved solids, the amount of polyvalentmetal ions present is not greater than about 6,000 ppm by weight.
 3. Amethod according to claim 2 wherein said total dissolved solids contentof said water is not greater than about 40,000 ppm by weight, and theamount of said polyvalent metal ions is not greater than about 3,000 ppmby weight.
 4. A method according to claim 2 wherein said polymer is asubstantially linear polymer of acrylamide.
 5. A method according toclaim 4 wherein said compound of a polyvalent metal is a compound ofchromium wherein the valence of the chromium is +6 and the valence of atleast a portion of said chromium is reduced to +3.
 6. A method accordingto claim 5 wherein said chromium compound is selected from the groupconsisting of ammonium chromate, ammonium dichromate, the alkali metalchromates and dichromates, chromium trioxide, and mixtures thereof.
 7. Amethod according to claim 6 wherein said reducing agent is selected fromthe group consisting of hydroquinone, sodium sulfide, sodiumhydrosulfite, sodium metabisulfite, potassium sulfite, sodium bisulfite,potassium metabisulfite, sodium sulfite, sodium thiosulfate, ferroussulfate, ferrous chloride, p-hydrazinobenzoic acid, hydrazine phosphite,hydrazine dihydrochloride, and mixtures thereof.
 8. A method accordingto claim 7 wherein: said chromium compound is sodium dichromate orpotassium dichromate; and said reducing agent is sodium hydrosulfite,potassium hydrosulfite, sodium thiosulfate, or potassium thiosulfate. 9,A Method according to claim 2 wherein: said polymer is a substantiallylinear polyacrylamide; said polyvalent metal compound is sodiumdichromate; and said reducing agent is sodium hydrosulfite or sodiumthiosulfate.
 10. A method according to claim 1 wherein: said polymer isselected from the group consisting of: polyacrylamides andpolymethacrylamides wherein from 0.1 to about 14 percent of thecarboxamide groups are hydrolyzed to carboxyl groups; crosslinkedpolyacrylamides and crosslinked polyacrylamides wherein from 0.1 toabout 14 percent of the carboxamide groups are hydrolyzed to carboxylgroups; copolymers of acrylamide with another ethylenically unsaturatedmonomer copolymerizable therewith, sufficient acrylamide being presentin the monomer mixture to impart said water-dispersible properties tothe resulting copolymer when it is mixed with water, and wherein from0.1 to about 14 percent of the carboxamide groups are hydrolyzed tocarboxyl groups; and mixtures of said polymers; and wherein the initialtotal dissolved solids content of said water is greater than about60,000 ppm by weight.
 11. A method according to claim 10 wherein saidaqueous gel comprises water to which there has been added: from 0.0025to 5 weight percent of said polymer, based upon the weight of saidwater; from 0.05 to 60 weight percent of said polyvalent metal compoundbased on the weight of said polymer; and from 0.1 to at least about 200percent of the stoichiometric amount of said reducing agent required toreduce said polyvalent metal to said lower polyvalent valence state. 12.A method according to claim 11 wherein said polymer is a substantiallylinear polymer of acrylamide.
 13. A method according to claim 12 whereinsaid compound of a polyvalent metal is a compound of chromium whereinthe valence of the chromium is +6 and the valence of at least a portionof said chromium is reduced to +3.
 14. A method according to claim 13wherein: said chromium compound is selected from the group consisting ofammonium chromate, ammonium dichromate, the alkali metal chromates anddichromates, chromium trioxide, and mixtures thereof; and said reducingagent is selected from the group consisting of hydroquinone, sodiumsulfide, sodium hydrosulfite, sodium metabisulfite, potassium sulfite,sodium bisulfite, potassium metabisulfite, sodium sulfite, sodiumthiosulfate, ferrous sulfate, ferrous chloride, p-hydrazinobenzoic acid,hydrazine phosphite, hydrazine dihydrochloride, and mixtures thereof.15. A method according to claim 10 wherein: said polymer is asubstantially linear polyacrylamide; said polyvalent metal compound issodium dichromate; and said reducing agent is sodium hydrosulfite orsodium thiosulfate.
 16. A method according to claim 1 wherein: saidformation is susceptible to attack by an acid; a slug of a fracturingfluid comprising said gel is injected into said formation; and a slug ofan acid is injected into said formation subsequent to the injection ofsaid fracturing fluid.
 17. A method according to claim 16 wherein a slugof a spacer fluid is injected into said formation after the injection ofsaid fracturing fluid and prior to injecting said acid.
 18. A methodaccording to claim 17 wherein said acid is selected from the groupconsisting of hydrochloric acid, formic acid, acetic acid, and mixturesthereof.
 19. A method according to claim 18 wherein said aqueous gelcomprises water to which there has been added: from 0.01 to 1.5 weightpercent of said polymer, based upon the weight of said water; from 0.5to 40 weight percent of said polyvalent metal compound, based upon theweight of said polymer; and from 0.1 to at least about 150 percent ofthe stoichiometric amount of said reducing agent required to reduce saidpolyvalent metal to said lower polyvalent valence state.
 20. A methodaccording to claim 19 wherein: said polymer is a substantially linearpolyacrylamide; said polyvalent metal compound is an alkali metaldichromate; and said reducing agent is sodium hydrosulfite or sodiumthiosulfate.
 21. A method according to claim 20 wherein: said polyvalentmetal compound is sodium dichromate; and said acid comprises a solutionof hydrochloric acid.
 22. A method according to claim 20 wherein: saidpolymer is a substantially linear polyacrylamide wherein from 0.1 toabout 14 percent of the carboxamide groups are hydrolyzed to carboxylgroups; said polyvalent metal compound is sodium dichromate; and saidacid comprises a solution of hydrochloric acid.
 23. A method accordingto claim 1 wherein the temperature of said formation is greater thanabout 200* F. and the life span of said gel is such that it breaks downto a viscosity approaching that of water in less than about 18 hours.24. A method according to claim 1 wherein an excessive amount of saidgelling agents is used relative to the amount of said polymer, so thatthe life span of said gel is such that it breaks down to a viscosityapproaching that of water in less than about 18 hours.
 25. A method offracturing a subterranean formation penetrated by a wellbore, whichmethod comprises, in combination, the steps of: A. forming a basefracturing fluid by adding to a given quantity of water from 0.01 to 1.5weight percent, based on the weight of said water, of a polymer selectedfrom the group consisting of: polyacrylamides and polymethacrylamideswherein up to about 75 percent of the carboxamide groups can behydrolyzed to carboxyl groups; crosslinked polyacrylamides andcrosslinked polyacrylamides wherein up to about 75 percent of thecarboxamide groups can be hydrolyzed to carboxyl groups; polyacrylicacid and polymethacrylic acid; polyacrylates; polymers of N-substitutedacrylamides wherein the nitrogen atoms in the carboxamide groups canhave from 1 to 2 alkyl substituents which contain from one to fourcarbon atoms; copolymers of acrylamide with another ethylenicallyunsaturated monomer copolymerizable therewith; sufficient acrylamidebeing present in the monomer mixture to impart said water-dispersibleproperties to the resulting copolymer when it is mixed with water, andwherein up to about 75 percent of the carboxamide groups can behydrolyzed to carboxyl groups; and mixtures of said polymers; B. addingto said base fracturing fluid one of (a) from 0.5 to 40 weight percentof a water-soluble compound of chromium wherein the valence of thechromium is +6 and which is sufficient to gel said water when thevalence of at least a portion of said chromium is reduced from +6 to +3,or (b) from 0.5 to at least about 150 percent of the stoichiometricamount of a water-soluble reducing agent which is effective to reducethe valence of said chromium from +6 to +3; C. pumping a slug of saidbase fracturing fluid of step (B) down said well and into said formationunder a pressure sufficient to create a fracture in said formation; andD. during said pumping adding to said base fracturing fluid the other ofsaid reagents (a) and (b) which was not added thereto in said step (B).26. A method according to claim 25 wherein: said polymer is asubstantially linear polyacrylamide; said chromium compound is sodiumdichromate or potassium dichromate; said reducing agent is sodiumhydrosulfite or sodium thiosulfate; and wherein said water has aninitial total dissolved solids content not greater than about 60,000 ppmby weight, and of said dissolved solids, the amount of polyvalent metalions present is not greater than about 6,000 ppm by weight.
 27. A methodaccording to claim 25 wherein: said polymer is a sUbstantially linearpolyacrylamide wherein from 0.1 to about 14 percent of the carboxamidegroups are hydrolyzed to carboxyl groups; said chromium compound issodium dichromate or potassium dichromate; said reducing agent is sodiumhydrosulfite or sodium thiosulfate; and wherein the initial totaldissolved solids content of said water is greater than about 60,000 ppmby weight.
 28. A method according to claim 25 wherein said formation issusceptible to attach by an acid, and said method comprises in furthercombination, the step of: E. injecting a slug of an acid into saidformation subsequent to the injection of said fracturing fluid as insaid steps (C) and (D).
 29. A method according to claim 28 wherein aslug of a spacer fluid is injected into said formation after said steps(C) and (D) and prior to said step (E).
 30. A method according to claim28 wherein: said polymer is a substantially linear polyacrylamide; saidchromium compound is sodium dichromate or potassium dichromate; saidreducing agent is sodium hydrosulfite or sodium thiosulfate; said acidcomprises a solution of hydrochloric acid; and wherein said water has aninitial total dissolved solids content not greater than about 60,000 ppmby weight, and of said dissolved solids, the amount of polyvalent metalions present is not greater than about 60,000 ppm by weight, and of saiddissolved solids, the amount of polyvalent metal ions present is notgreater than about 6,000 ppm by weight.
 31. A method according to claim28 wherein: said polymer is a substantially linear polyacrylamide; saidchromium compound is sodium dichromate or potassium dichromate; saidreducing agent is sodium hydrosulfite or sodium thiosulfate; said acidcomprises a solution of hydrochloric acid; and wherein the initial totaldissolved solids content of said water is greater than about 60,000 ppmby weight.